compound 2a was spread on a piece of black cardboard (or
nonbleached filter paper). Upon exposure to long-wave UV
light, a bright yellow fluorescence was apparent. After dipping
the strip in liquid nitrogen for 10−20 s, the fluorescence was
bright violet. The spot gradually changed to red-orange and
then back to yellow as the solid warmed to room temperature
(Figure 3). Compounds 2b−2e showed different color